A NEW CLASS OF X-RAY STAR?

Teaming up space telescopes to make simultaneous ultraviolet
and X-ray observations, astronomers may have solved a
20-year old mystery and possibly discovered a new class
of X-ray star.

The unlikely suspect is a second magnitude star 600 light-years
away in the middle point of the "W" in circumpolar constellation
Cassiopeia. It turns out the mild-mannered looking star is
ejecting 100 million degree flares into space -- 10 times hotter
than typical flares ejected from our sun.

The results are being announced today to the 191st meeting of the
American Astronomical Society in Washington, DC by Myron Smith
of the Space Telescope Science Institute in Baltimore, MD. His
co-investigators are Richard Robinson and Robin Corbet of the
Goddard Space Flight Center in Greenbelt, MD.

The team pointed both the Hubble Space Telescope and the Rossi
X-Ray Timing Explorer at the hot star gamma Cassiopeiae,
simultaneously for one full day in March, 1996 in order to
understand the origin of the bright, variable X-rays coming from
this star. To their surprise they discovered the X-rays are probably
produced by extraordinarily hot surface flares, which are
completely unexpected and unpredicted for a star of this type.

A New Tack On A 20-Year Mystery

In the 1860's an Italian priest, Father Secchi, discovered that
gamma Cassiopeiae shows bright emission in the hydrogen lines of
its spectrum. His finding made this star the first member of the
class of "B-emission" (Be) stars. These stars are hotter and
several times more massive than the Sun and are known for their
unexplained eruptions of mass from time to time.

Twenty years ago astronomers had discovered that surprisingly
bright X-ray light came from this well-known star, there has
been no consensus since then on the reason for these bright
emissions.

Most "Be" stars generally show only faint X-ray emissions,
similar to other hot stars. The X-ray emissions of gamma
Cassiopeiae stand out because they originate from gas which is hot
even by astronomical standards, 100 million degrees Kelvin. The
emissions also show a zig-zag variability over only several
seconds which is reminiscent of X-ray flares on the Sun and
similar low-mass, "cool" stars.

Previous explanations of this X-ray behavior have suggested that
the energy is created by the infall of gas onto a dense companion
star, such as a neutron star. However, despite repeated searches
no evidence has been found for a companion star.

The simultaneous observations of the X-ray and Hubble
telescopes dramatically changed this view by showing that the
slower X-ray variability in gamma Cassiopeiae is consistent with
its ultraviolet variations. Comparison of these slow undulations
with past X-ray observations suggest that they repeat with a
period of 27 hours. The Hubble observations also indicate that
gas lobes hang over the star, which is also consistent with a
magnetic flare interpretation.

Confirming evidence comes from a repeated 27 hour period between
the Hubble observations and ultraviolet variations of this star
observed by the International Ultraviolet Explorer satellite two
months earlier. Since a period of 27 hours is consistent with
that expected with the star's rotational spin rate, Smith and
his team believe the X-rays originate from the star's surface.

Though relatively cooler flares are common on the Sun, it is
surprising that they occur on a Be-class star. Solar flares are
caused by magnetic fields inside the Sun which then get tangled
and finally cause violent explosions on the surface.

In contrast, the theory for massive stars predicts that they
should not generate such fields. "Thus, at the present time, the
precise mechanism that creates these X-rays is still unclear,"
Smith concludes.

Despite this uncertainty, Smith points out "there is no such
thing as uniqueness in astronomy. Where there is one peculiar
star, there are many more somewhere else. There is probably an
entire class of these X-ray variables out there."

Robinson notes that "with the new generation of X-ray satellites
recently or soon to be launched, it should be possible to
discover a class of fainter B-emission stars that are variables
just like gamma Cassiopeiae."

The Space Telescope Science Institute is operated by the
Association of Universities for Research in Astronomy, Inc.
(AURA) for NASA, under contract with the Goddard Space
Flight Center, Greenbelt, MD. The Hubble Space Telescope
is a project of international cooperation between NASA and
the European Space Agency (ESA).

FIGURE CAPTION: This plot shows the similarity of X-ray and ultraviolet
light emissions from the B-emission star gamma Cassiopeiae on
March 14--15, 1996. The UV emission from the HST has been
inverted and rescaled to emphasize its correspondence with the
slow component of the variations of the X-ray emission.